Methods of the type mentioned at the outset are believed to be understood from the related art. These methods are supposed to prevent the wheel from locking on a road surface beneath the vehicle in such a way that the wheel slip exceeds a certain value starting from which the locking tendency occurs. In this way, it should be prevented that the wheel gets too far into the sliding friction range, on the one hand, and the vehicle should remain controllable, on the other hand. The latter is not the case if the locking wheel is used to steer the vehicle, since no lateral cornering forces may be transferred to the road surface in this case. For this reason, during the braking operation of the vehicle, which is induced by a driver of the vehicle or by a driver assistance system, for example, the braking force variable set at the braking device should be increased until the locking tendency is recognized.
The locking tendency is present when the wheel slip exceeds a certain limiting value. For this purpose, the wheel may be assigned an arrangement for determining its rotational speed, which may be compared to a reference velocity of the vehicle or to a reference rotational speed. Upon recognizing the locking tendency, the braking force variable is reduced to the certain or predetermined value. This value equals zero, for example, or corresponds to a braking force variable at which the locking tendency is no longer present or is no longer detected. The braking system of the vehicle is thus an ABS braking system. It usually has an ABS control unit which may be designed as a controller. The braking force variable may be, for example, directly the set braking force or, alternatively, a brake pressure, a brake slip or the like.
This controller may, for example, work according to the optimizer principle. This controller is also often referred to as an instability controller. It sets the braking force variable acting on the wheel in such a way that the previously described succession of increasing the braking force variable until the locking tendency is recognized and reducing it to a certain value is implemented. The maximum of a μ-slip curve is cyclically surpassed and the braking force variable of the wheel thus approaches a maximum. The μ-slip curve is a curve which describes the behavior of the friction coefficient μ plotted against the wheel slip. The μ-slip curve therefore depends on the wheel and on the road surface beneath the vehicle, and on the surrounding conditions (temperature, humidity and the like).
Controllers for a rear axle of the vehicle, which work according to the optimizer principle, are often additionally superimposed with a select-low control, or a pressure difference control, in order to keep the side-force reserves for a lateral cornering of the vehicle (driving stability) in an admissible range. In the select-low control, the wheel having the greatest locking tendency determines a common braking force variable of the wheels associated with the rear axle. Each of the wheels thus has arrangement(s) for determining the rotational speed and the braking device, the braking force variable set at this braking device being the same for all wheels of the rear axle.
Controllers are also understood to work according to the slip control principle, i.e., they set the wheel slip to a certain value. Such a controller requires a very precise reference speed which, however, may only be determined with the aid of additional sensors which are adequately precise. These additional sensors must be provided in addition to the arrangement for determining the rotational speed of the wheel. The disadvantages of the procedures described above are that either additional sensors are needed, or the traveling comfort for the driver of the vehicle is reduced due to strong cyclic difference pressures or very strong oscillations of the braking force variable.